I made a game and I put a skybox or simply box but when I start game the camera isn't displaying it
There is a Very simple reason why your sky box is not displaying. The reason for this is that you did not reverse the normals on your cube.

If you enable Backface Culling and set the normals to display,



You will notice that your normals are pointing outward. This means that you can see them from the outside.

If you reverse the normals, (Select the faces that are pointing the wrong way and click Flip Normals 



it will make the normals point inward.



Now you are seeing through the faces pointing inward to the other side.
If you were inside the cube, you will see all the faces properly.